Perfect timing - a bit too strong?

It gives you a free 300 gold lead, you don't have to back to buy it, and on top of that it gives hourglass items a boost. Since the increased cost is added on at full build, you essentially gets a 600 gold lead at the 6th minute, down to 300 when you builds the final item. Seems way too strong, since it's available as a secondary path mastery.

6 minute mark free zhonya would also allows you to worry less about backing as you have a much larger outplay potential in case of a dive. In case your opponent didn't notice, you can also bait them for a kill.

Proposed change: you have to reach fountain after 6th minute to activate the commencing stopwatch, and not have it being ready while you're still in lane.